Kurt J. Schuster

Infrared skin damage thresholds from 1940-nm continuous-wave laser exposures

Abstract. A series of experiments were conducted in vivo using Yucatan miniature pigs (Sus scrofa domestica) to determine thermal damage thresholds to the skin from 1319-nm continuous-wave Nd:YAG laser irradiation. Experiments employed exposure durations of 0.25, 1.0, 2.5, and 10 s and beam diameters of ∼0.6 and 1 cm. Thermal imagery data provided a time-dependent surface temperature response from the laser. A damage endpoint of fifty percent probability of a minimally visible effect was used to determine threshold for damage at 1 and 24 h postexposure. Predicted thermal response and damage thresholds are compared with a numerical model of optical-thermal interaction. Resultant trends with respect to exposure duration and beam diameter are compared with current standardized exposure limits for laser safety. Mathematical modeling agreed well with experimental data, predicting that though laser safety standards are sufficient for exposures <10  s, they may become less safe for very long exposures.

Trends in melanosome microcavitation thresholds for nanosecond pulse exposures in the near infrared

Abstract. Thresholds for microcavitation of bovine and porcine melanosomes were determined using nanosecond laser pulses in the near-infrared (1000 to 1319 nm) wavelength regime. Isolated melanosomes were irradiated by single pulses (10 or 50 ns) using a Q-switched Spectra Physics Nd:YAG laser coupled with an optical parametric oscillator (1000 to 1200 nm) or a continuum laser at 1319 nm. Time-resolved nanosecond strobe photography after the arrival of the irradiation beam allowed imaging of microcavitation events. Average fluence thresholds for microcavitation increased nonlinearly with increasing wavelength from ∼0.5  J/cm2 at 1000 nm to 2.6  J/cm2 at 1319 nm. Fluence thresholds were also measured for 10-ns pulses at 532 nm and found to be comparable to visible nanosecond pulse values published in previous reports. Calculated melanosome absorption coefficients decreased from 925  cm−1 at 1000 nm to 176  cm−1 at 1319 nm. This trend was found to be comparable to the decrease in retinal pigmented epithelial layer absorption coefficients reported over the same wavelength region. Estimated corneal total intraocular energy retinal damage threshold values were determined in order to compare to current and proposed maximum permissible exposure (MPE) safe levels. Results from this study support recently proposed changes to the MPE levels.

In-vitro Retinal Model Reveals a Sharp Transition between Laser Damage Mechanisms

We use laser damage thresholds in an in-vitro retinal model, and computational simulations to examine the laser exposure durations at which damage transitions from photothermal to photochemical at 413 nm. Our results indicate a dramatic shift in 1-h damage thresholds between exposure durations of 60 and 100 s. The trend in our in-vitro results is similar to a trend found in a recent study where retinal lesions were assessed 1-h post laser exposure in the rhesus eye Our data suggest that nonthermal mechanisms did not significantly contribute to cell death, even for exposures of 60 s. Knowledge of the transition point, and lack of concurrent thermal and nonthermal damage processes, are significant for those wishing to devise a comprehensive computational damage model.

Effect of laser thermal injury on Langerhans cells in mouse and hairless guinea pig epidermis.

To examine the effect of laser thermal injury on Langerhans cells (LC) within the epidermis, the dorsal skin of mice and hairless guinea pigs was exposed to varying levels of laser irradiation using a thulium laser at a wavelength of 2.0 μm. At 6, 24 and 48 h post irradiation, animals were euthanized, skin samples prepared for histology and the epidermis obtained and stained by major histocompatibility complex‐II staining (mice) or ATPase assay (hairless guinea pigs) for the enumeration of LC. Mouse skin exhibited histological evidence of thermal damage at 24 h post irradiation at even the lowest dose (0.14 W) and decreases in the numbers of epidermal LC were observed at all doses and decreases were proportional to dose. In contrast, hairless guinea pig skin only showed consistent histological evidence of thermal damage at the highest dose of irradiation (0.70 W) at 24 and 48 h post irradiation and exhibited a statistically significant decrease in numbers of epidermal LC only at this dose. Thus, epidermal LC depletion occurred in the skin of both mice and hairless guinea pigs in response to laser treatment and the magnitude of depletion directly correlated with the extent of thermal damage both within and between species.

Porcine skin damage thresholds for 0.6 to 9.5 cm beam diameters from 1070-nm continuous-wave infrared laser radiation.

Abstract. There is an increasing use of high-power fiber lasers in manufacturing and telecommunications industries operating in the infrared spectrum between 1000 and 2000 nm, which are advertised to provide as much as 10 kW continuous output power at 1070 nm. Safety standards have traditionally been based on experimental and modeling investigations with scant data available for these wavelengths. A series of studies using 1070-nm infrared lasers to determine the minimum visible lesion damage thresholds in skin using the Yucatan miniature pig (Sus scrofa domestica) for a range of beam diameters (0.6, 1.1, 1.9, 2.4, 4.7, and 9.5 cm) and a range of exposure durations (10 ms to 10 s) is presented. Experimental peak temperatures associated with each damage threshold were measured using thermal imaging. Peak temperatures at damage threshold for the 10-s exposures were ∼10°C lower than those at shorter exposures. The lowest and highest experimental minimum visible lesion damage thresholds were found to have peak radiant exposures of 19 and 432  J/cm2 for the beam diameter-exposure duration pairs of 2.4 cm, 25 ms and 0.6 cm, 10 s, respectively. Thresholds for beam diameters >2.5  cm had a weak to no effect on threshold radiant exposure levels for exposure times ≤0.25  s, but may have a larger effect on thresholds for exposures ≥10  s.

Skin damage thresholds with continuous-wave laser exposures at the infrared wavelength of 1319 nm

ABSTRACT Damage thresholds (ED50) for skin using Yucatan mini-pig (Sus scrofa domestica) have been determined at the operational wavelength of 1319 nm with beam diameters of 0.61 cm and 0.96 cm. Exposure durations of 0.25, 1.0, 2.5 and 10 seconds were used to determine trends in damage threshold with respect to exposure time and beam diameter at this moderately-high penetrating wavelength. A relatively narrow range of total radiant exposure from 37.4 J/cm2 to 62.3 J/cm2 average was observed for threshold damage with laser parameters encompassing a factor of two in beam area and a factor of forty in exposure duration.

Exposing human retinal pigmented epithelial cells to red light in vitro elicits an adaptive response to a subsequent 2-μm laser challenge

The objective of this study was to elucidate cellular mechanisms of protection against laser-induced thermal killing utilizing an in vitro retina model. When exposed to a 1-sec pulse of 2-μm laser radiation 24 hr after illuminating hTERT-RPE cells with red light (preconditioning), the cells are more resistant to thermal challenge than unilluminated controls (adaptive response). Results of efforts to understand the physiology of this effect led us to two genes: Vascular Endothelial Growth Factor C (VEGF-C) and Micro RNA 146a (miR-146a). Transfecting wild type (WT) cells with siRNA for VEGF-C and miR-146a mRNA resulted in knockdown strains (VEGF-C(KD) and miR- 146a(-)) with 10% and 30% (respectively) of the constitutive levels expressed in the WT cells. To induce gene expression, WT or KD cells were preconditioned with 1.44 to 5.40 J/cm2, using irradiances between 0.40 and 1.60 mW/cm2 of either 671-nm (diode) or 637-nm (laser) radiation. Probit analysis was used to calculate threshold damage irradiance, expressed as ED50, between 10 and 100 W/cm2 for the 2-μm laser pulse. In the WT cells there is a significant increase in ED50 (p 0.05) with the maximum response occurring at 2.88 J/cm2 in the preconditioned cells. Neither KD cell strain showed a significant increase in the ED50, although some data suggest the response may just be decreased in the knockdown cells instead of absent. So far the response does not appear to be dependent upon either wavelength (637 vs. 671 nm) or coherence (laser vs. LED), but there is an irradiance dependence.

VEGF-C as a survival factor for retinal pigment epithelial cells from photothermal stress

Vascular endothelial growth factor (VEGF) is known for its role in neovascularization and cellular signaling pathways of sub-threshold retinal lesions. The objective of this study was to elucidate potential protection mechanisms to laser-induced heat stress utilizing an in vitro retinal model. The cell line was characterized to determine the relative abundance of VEGF-C protein. Cells, preconditioned via water bath and controls, were then exposed to 2 μm laser radiation to assess whether increases in protein production following preconditioning could confer any protection. There was no significant increase in threshold damage irradiance (ED50) in the preconditioned cells versus control.

Melanin microcavitation threshold in the near infrared

Thresholds for microcavitation of isolated bovine and porcine melanosomes were determined using single nanosecond (ns) laser pulses in the NIR (1000 – 1319 nm) wavelength regime. Average fluence thresholds for microcavitation increased non-linearly with increasing wavelength. Average fluence thresholds were also measured for 10-ns pulses at 532 nm, and found to be comparable to visible ns pulse values published in previous reports. Fluence thresholds were used to calculate melanosome absorption coefficients, which decreased with increasing wavelength. This trend was found to be comparable to the decrease in retinal pigmented epithelial (RPE) layer absorption coefficients reported over the same wavelength region. Estimated corneal total intraocular energy (TIE) values were determined and compared to the current and proposed maximum permissible exposure (MPE) safe exposure levels. Results from this study support the proposed changes to the MPE levels.

The response of human retinal pigmented epithelial cells in vitro to changes in nitric oxide concentration stimulated by low levels of red light

The goal of this project is to explore the role of nitric oxide (NO) in regulating the response of hTERT-RPE to low-level exposures to red light. Exposure to low-level red light has been shown to positively affect wound healing, reduce pain, and encourage cell proliferation. The current explanation for this effect is described as an interaction between the photons and cytochrome c oxidase (Cco), which plays a role in regulation of intracellular NO levels in addition to being the mitochondrial protein complex where reduction of oxygen occurs in the process of oxidative phosphorylation. Exposure to 2.88 J/cm2 of 671-nm and 637-nm light shows a two-fold increase in NO immediately after exposure, and a 56% increase in ATP measured at ~5 h post exposure. Levels of NF-κB mRNA and protein were measured at six and 24 h, respectively, and found to increase six fold, correlating with increases in NO levels. Light-stimulated increased levels of NO also correlated with an 11-fold increase in Bcl-2 and a 70% decrease in Bax mRNA levels, relative to controls. NF-κB promotes cell growth and Bcl-2 is an apoptosis suppressor protein. Bax is a positive apoptotic effector protein. These results support the hypothesis that light-induced changes in the intracellular levels of NO play a role in the beneficial effects of low-level light photobiomodulation